They will evaluate upcoming discussions in the Kazakh capital Astana aimed at reviving the tattered ceasefire, and prospects for convening talks in Geneva between Syria's warring sides in May, de Mistura said.

"The trilateral meeting - as you know that was a possibility - is being postponed, it is not taking place on Monday. It will be a bilateral... But the trilateral is not off the table, it is just being postponed," he told reporters.

De Mistura, asked about the US administration's intent to participate, replied: "There is a clearly an intention to maintain and resume these trilateral meetings, and the date and circumstances were not conducive for this to happen on Monday."

The United States carried out an air strike on a Syrian air base earlier this month after a chemical weapons attack that killed scores of people near Idlib on April 4. Both incidents have raised tensions between Washington and Moscow, the Syrian government's ally.

"The US welcomes discussions with Russia regarding the UN-led political process on Syria in Geneva, and we have met in the past in the trilateral US-Russia-UN format. But at this time there is no new trilateral meeting scheduled," a US spokesman in Geneva said in an email to Reuters in response to a query.

De Mistura said that his team had just taken part in technical talks in Tehran, in preparation for the Astana talks organised by Russia, Iran and Turkey.

There had been "some type of movement regarding the issue of detainees ...and on possible issues related to demining as well," he said.

All sides have flagged their readiness to allow aid convoys to reach Douma and later other besieged towns in the eastern Ghouta province near Damascus, UN humanitarian adviser Jan Egeland said. "It is very, very urgent that we get to eastern Ghouta."

Egeland added: "In eastern Ghouta there are nearly 400,000 people that are now besieged, they are in crossfire and they are without supplies. They are in a desperate situation."